Q:
How can black holes that are as massive as you describe be positive influences in the cosmos

A:
It depends on what you mean by "positive influence." If you mean a positive influence for the carbon units we call humans, we can identify several positive influences in order of declining certainty.

First of all, the study of black holes provides many hours of fascinating work for astronomers and physicists.

Second, the concept of black holes has given us a very useful metaphor for describing lots of things that seem to get lost: money, socks, ballpoint pens, time, golf balls, etc. Instead of getting lost down a rat hole, they now get lost down a black hole. This may be more positive for rats than humans.

Farther afield and less direct, black holes are apparently formed as part of the supernova process when a massive star explodes. These explosions distribute elements such as carbon, nitrogen, oxygen, etc. that are necessary for life into space. The shock waves from the explosions may also trigger the formation of new stars and new solar systems, so our existence on Earth might be due to an explosion that formed a black hole.

On a larger scale, most galaxies seem to have supermassive black holes (millions of times as massive as the Sun) located at their centers. The connection between the formation of these supermassive black holes and the formation of galaxies is still not understood. The possibility exists that a black hole could have played a role in the formation of our Milky Way galaxy.

Now for the real wild stuff, sometimes referred to as black hole magic:

Some scientists have speculated that black holes form bridges connecting widely separated regions of space-time. Such bridges, sometimes called wormholes, are very popular in the entertainment industry, and could supposedly be used for faster-than-light travel in space. The construction of a workable wormhole requires an as-yet undiscovered source of exotic (anti-gravity) matter to hold the wormhole open. (Ref: K. Thorne, Black Holes and Time Warps)

Finally, it has been suggested that black holes are the birthplaces of other universes. The idea is that each black hole collapses into a singularity or infinitely dense point and then inflates in another dimension to form a new or offspring universe. Black holes in this offspring universe would give rise to still more universes. Lee Smolin proposed that this would form the basis of a natural selection theory that would explain why the forces of gravity, electromagnetism, etc. have the strengths they do in our universe, which are compatible with the evolution of life in the universe. (Ref: Lee Smolin, The Life of the Cosmos)